Cryogenic liquids are liquefied gases that generally have boiling points below −100° C. (about −150° F.) at atmospheric pressure. Examples of cryogenic liquids include liquid natural gas (LNG), nitrogen, oxygen, carbon dioxide, methane and hydrogen.
Cryogenic liquids are usually stored in thermally insulated storage tanks that consist of an inner storage vessel mounted within an outer shell, with thermal insulation provided by insulating materials and a vacuum disposed in the space between the inner vessel and the outer shell. The inner vessel defines the cryogen space in which a cryogenic liquid can be stored. Such an arrangement reduces the transfer of heat from the ambient environment to the cryogenic liquid stored within the cryogen space. Such heat transfer is also known as “heat leak” and it is desirable to reduce heat leak into the cryogen space. Heat leak results in vaporization of cryogenic liquid, which in turn results in raising the vapor pressure in the cryogen space. If the vapor pressure in the cryogen space rises above the set point of the pressure relief valve, vapor is vented to atmosphere. To avoid wasting cryogenic liquid through venting, and the consequent release of the stored fluid into the environment, it is desirable to design storage tanks to reduce heat leak so that cryogenic liquids can be stored for longer periods of time without venting. Each pipe that penetrates through the insulating space and into the cryogen space provides a thermal conduction path that can contribute to heat leak.
Conventional storage tanks for cryogenic liquids employ separate pipes for filling and venting the cryogen space. Vent pipes are provided with a pressure relief valve that opens if the vapor pressure within the cryogen space exceeds a predetermined set point, selected based on the designed pressure rating of the storage tank. Fill pipes and vent pipes can both have open ends in the upper portion of the cryogen space that is normally occupied by vapor. For reasons of safety, design codes typically require an arrangement that prevents cryogenic fluid from escaping from the storage tank through the fill pipe, and conventional fill pipes typically employ a check valve for this purpose. Consequently, with conventional storage tanks, this check valve makes the fill line unsuitable for use as a vent line.
For example, U.S. Pat. No. 5,572,874, entitled, “Relief Valve Construction to Minimize Ignition Hazard from Cryogenic Storage Tanks Containing Volatile Liquids” discloses an arrangement for venting from the cryogen space of a storage tank for cryogenic liquids. In this example, one or more dedicated vent pipes are employed with respective pressure relief valves. It is common in some storage tanks to employ two pressure relief valves in case one of the valves malfunctions or in case the vent line associated with one of the pressure relief valves becomes blocked, hi such arrangements, the set point for opening the pressure relief valves can be higher for one of the pressure relief valves.
U.S. Pat. No. 4,956,975, entitled, “Shutoff Valve for Cryogenic Liquid Storage Tank” discloses another arrangement for a vent line that comprises a shutoff valve in the vent line that is closed by the momentum of the flow of cryogenic liquid when the storage tank is filled to its maximum capacity. Like other known storage tanks for cryogenic liquids, a dedicated pipe is employed as a vent line. In the disclosed arrangement one pipe is employed for filling and withdrawing cryogenic liquid from the bottom of the cryogen space. A disadvantage of filling through such a pipe is that when filling the storage tank, the incoming cryogenic liquid is not as effective at condensing vapor that might be inside the cryogen space. Preferred designs employ fill pipes that open into the upper part of the cryogen space where the cryogenic liquid can be sprayed to condense vapor and reduce vapor pressure, allowing faster and/or more efficient refilling.
When beginning to re-fill a storage tank with cryogenic liquid, the fill line may initially be at a temperature that causes some of the cryogenic liquid to be vaporized until the cryogenic liquid cools the fill line. It would be an improvement to reduce the amount of cryogenic liquid that is vaporized while cooling the fill line.